The invention relates generally to the field of optical pulse generation. In particular, the invention relates to apparatus for narrow pulse generation and methods of generating narrow pulses.
Narrow optical pulse generation is required for numerous communications and sensor systems. Narrow optical pulses are optical pulses that occupy small intervals of time or optical pulses that have a steep intensity change produced by a control signal. In telecommunications systems, for example, the transmission of optical pulses is used when the modulation format requires that the intensity change from off to on, and then off again within a bit time period. This produces pulses of light, which comprise clocking or data signals.
Return-to-Zero (RZ) data refers to data which is either off or on for approximately half the bit period. Non-Return-to-Zero (NRZ) data refers to data where the light is on or off for the whole bit period. FIG. 1 illustrated a prior art timing diagram 10 of Clock 12, NRZ data 14, and RZ data format 16. Typically, these data formats can be constructed in the electrical system by using a logical xe2x80x9cand ingxe2x80x9d between the data clock and the data itself.
At high data rates, it is difficult to generate pulses electrically with prior art optical modulators. It is also difficult to generate pulses having a predetermined shape for the specific application such as soliton and other narrow optical pulse formats for very long distance propagation.
There exists several prior art pulse generators for generating narrow and predetermined pulse formats that comprise cascaded replications of Mach-Zehnder interferometers. These prior art devices use separately fed controlling sections. The input signals and operating bias state of the aggregate device is controlled in a variety of ways depending on the design. Some of these designs use modified input signals to each section of the aggregate device to produce the desired pulse train. Other prior art methods partially modulate the transfer function of a modulator with a device, such as an electro-absorption modulator, in order to generate fast pulses.
There are numerous disadvantages of these prior art designs. For example, these methods require precise control of the time delay and phase of the different input signals, which is both difficult and costly to achieve. Also, there is a relatively high power penalty associated with generating a number of high-speed signals and associated with the additional physical length required for the device.
There exists a need for an apparatus and method for generating narrow RZ pulses for modern communications systems. There also exists a need for generating pulses with a very narrow width that can be transmitted over long distances. There also is a need for an apparatus and method for generating a Gaussian, or hyperbolic secant squared shape pulse at high speeds, which is the algebraic shape required for soliton pulse generation.
The present invention relates to a pulse generator comprising a high order function waveguide interferometer that generates narrow pulses and pulses having a predetermined shape for specific applications such as soliton and other narrow optical pulse formats. A principal discovery of the present invention is that nested and parallel configurations of interferometric modulators can be used to generate narrow pulses and pulsing having a predetermined shape for specific applications.
Accordingly, the present invention features an optical pulse generator having a high order transfer function. In one embodiment, the optical pulse generator includes a first and a second nested interferometric modulator, each modulator comprising an optical input, an electrical input, a first arm, a second arm and an optical output. The second interferometric modulator is optically coupled into the second arm of the first interferometric modulator. The optical output of the first interferometric modulator generates pulses at a repetition rate that is proportional to a multiple of a frequency of an electrical signal applied to the electrical input of at least one of the first and second interferometric modulator and at a duty cycle that is inversely proportional to the order of the transfer function of the optical pulse generator. The duty cycle may be inversely non-linearly monotonically proportional to the order of the transfer function of the optical pulse generator. The multiple may be any integer equal to or greater than one. A phase modulator may be coupled in series with the output of the first interferometric modulator to chirp the optical pulses with a modulation signal applied to an electrical input of the phase modulator.
In one embodiment of the invention, the pulse generator also includes a third interferometric modulator comprising a first and second arm and an electrical input. The third interferometric modulator has an input optically coupled to the output of the first interferometric modulator. The pulse generator of this embodiment also includes a fourth interferometric modulator comprising a first and second arm and an electrical input. The fourth interferometric modulator is optically coupled into the second arm of the third interferometric modulator. The optical output of the third interferometric modulator generates pulses at a repetition rate that is proportional to a multiple of a frequency of an electrical signal applied to the electrical input of at least one of the second and the fourth interferometric modulator and at a duty cycle that is inversely non-linearly proportional to the order of the transfer function of the optical pulse generator.
The interferometric modulators may be amplitude or phase modulators. In one embodiment, the interferometric modulators are Mach-Zehnder modulators formed on a lithium niobate substrate that may be X-cut or Z-cut. The interferometric modulators may also be substantially velocity matched or substantially temperature compensated.
In one embodiment of the invention, the interferometric modulators are narrow band modulators. That is, the bandwidth of the modulators is substantially limited to a predetermined bandwidth. Using narrow band modulators may increase the efficiency of the optical pulse generation. In one embodiment, the splitting ratio between the first and the second arm of at least one interferometric modulator is substantially less than one.
The present invention also features an optical pulse generator having a high order transfer function that comprises a plurality of interferometric modulators optically connected in parallel. Each of the plurality of interferometric modulators includes a first and second arm and an electrical input. The optical pulse generator having even order transfer functions includes an optical waveguide that is optically coupled in parallel with the plurality of interferometric modulators. An optical output generates optical pulses having a repetition rate that is proportional to a multiple of a frequency of an electrical signal applied to the electrical input of at least one of the plurality of interferometric modulators and having a duty cycle that is inversely non-linearly proportional to the order of the transfer function of the optical pulse generator. The multiple may be any integer equal to or greater than one.
The output waveguide of at least one of the plurality of interferometric modulators may include a bias electrode, wherein a voltage applied to the bias electrode modifies a phase of an optical signal propagating from the at least one of the plurality of interferometric modulators. In addition, a phase modulator may be coupled in series with the output of the first interferometric modulator to chirp the optical pulses with a modulation signal applied to an electrical input of the phase modulator.
The present invention also features a method for generating optical pulses with a high order nested interferometric modulator. The method may generate narrow pulses and pulses having a predetermined shape for specific applications such as soliton and other narrow optical pulse formats. The method includes receiving an input optical beam and splitting the beam into a first and second optical beam. A material propagating the first optical beam is electro-optically biased, thereby changing a characteristic of the first optical beam. The electro-optical bias may change the extinction ratio of the pulses.
The second optical beam is split into a third and fourth optical beam. A material propagating at least one of the third and the fourth optical beams is electro-optically biased thereby changing a characteristic of at least one of the third and the fourth optical beams. At least one of the third and fourth optical beams is modulated with an electrical signal. The first, third, and fourth optical beams are interfered to generate optical pulses having a repetition rate that is proportional to a multiple of a frequency of the electrical modulation signal and having a duty cycle that is inversely non-linearly proportional to the order of the nested interferometric modulator.